1. Field of the Invention
The invention relates to a protection circuit for a plurality of fans, a cooling system comprising such a protection circuit, and a display apparatus comprising such a cooling system.
2. Description of the Related Art
Japanese Patent Application No. JP-A-61-15594 discloses fans which are each connected to an operating voltage via a series arrangement of a current sensor and a breaker. A comparison calculator compares, for each fan, the actual fan current as measured by the corresponding current sensor with a normal operating current. If the difference between the actual fan current and the normal operating current exceeds a prescribed allowable level, the corresponding breaker is opened. This fan protection device has the drawback that a conductive line is required from each current sensor to the comparison calculator to provide the actual fan currents.
It is an object of the invention to provide a protection circuit for a plurality of fans, wherein the number of conductive lines required to provide the actual operating status of the fans to a detection circuit does not depend on the number of fans.
To this end, a first aspect of the invention provides a protection circuit for a plurality of fans, the protection circuit comprising a plurality of elements, each element being associated with a corresponding one of the plurality of fans and having a property with a value depending on an operation condition of the corresponding one of said fans, the elements being arranged in parallel between a reference line and a protection line, and a detection circuit coupled to the protection line for detecting whether a total value of the parallel-arranged elements is in a range indicating that at least one of the fans is in an abnormal operation condition.
A second aspect of the invention provides a cooling system comprising a plurality of fans and a protection circuit as described above.
A third aspect of the invention provides a display apparatus having a plurality of fans and a protection circuit as described above.
In the protection circuit for a plurality of fans in accordance with the first aspect of the invention, a circuit (further referred to as the element or elements) indicating the operation condition of a corresponding fan, is associated with each fan. Each element has a property with a value which indicates whether the corresponding fan is operating normally or abnormally.
The elements are arranged in parallel between two conductive lines. The detection circuit determines the total value of the properties of parallel-arranged elements. If the total value is not within a predetermined range, which indicates that all the fans are operating normally, at least one of the fans functions abnormally. The number of lines required to convey the operation status of the fans to the detection circuit is only two and does not depend on the number of fans involved.
The protection circuit in accordance with the invention has the further advantage that the total value may indicate how many fans are not functioning properly. For example, if six fans are used, it may be decided to take action only if two or more fans are operating abnormally. In the prior art, all fans will be switched off when a single fan operates abnormally. The protection circuit may protect overheating of an apparatus if one or more fans are operating abnormally.
Japanese Patent Application No. JP-A-2-230411 discloses a system for detecting fan abnormality, wherein a fuse opens when the corresponding fan operates abnormally. All the fuses are arranged in series. One end of the series arrangement is connected to an input of a detector. A pull-up resistor is connected to the input of the detector. If one of the fans operates abnormally, the corresponding fuse opens the series chain of fuses and the input will be pulled to a high voltage by the pull-up resistor. This prior art does not disclose a parallel arrangement of the elements, and the detection circuit does not check the value of the properties of the parallel-arranged elements. Moreover, this prior art is unable to detect how many fans are functioning abnormally as it cannot be distinguished whether a single fan or more fans is or are operating abnormally.
In an embodiment of the subject invention, the element comprises a current source which supplies a current depending on the operation condition of the corresponding fan. The total current caused by the parallel-arranged current sources may be measured directly or converted into a voltage via a common impedance connected to the protection line. The measured current or voltage can be used to determine whether one or more fans is or are inoperative. For example, let it be assumed that the current sources do not supply current as long as the fans operate normally, and each current source produces a predetermined amount of current if a corresponding fan operates abnormally. The number of times that the predetermined amount of current appears in the total current indicates the number of fans that are inoperative.
In another embodiment of the invention, the current-determining element comprises an impedance element whose impedance value depends on the operation condition of the corresponding fan. The detection circuit determines the total impedance of the parallel-arranged impedance elements. If the total impedance is not within a predetermined range, which indicates that all the fans are operating normally, at least one of the fans functions abnormally.
In another embodiment of the invention, the impedance element comprises an impedance in series with a switch to decrease the tolerance of the measured impedance.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.